Improving the Position Accuracy In Archived HST Images

Dencheva, Nadezhda

We present changes to the HST pipeline, which aim at increasing the astrometric accuracy of archived HST images through successive WCS corrections. In addition, all distortion information and astrometric corrections are stored in the science files, decreasing the size of a typical ACS observation download in half.

We have developed two software packages to support these changes: PyWCS and STWCS. PyWCS is a general purpose WCS library which provides python bindings for WCSLIB and an implementation of WCS Paper IV Lookup Table and Simple Imaging Polynomial (SIP) conventions. STWCS extends PyWCS and defines an HST specific WCS object which incorporates the distortion, velocity aberration and detector specific geometry calibrations in the science files. STWCS also provides tools for handling alternate WCS descriptions. WCS corrections utilize the Simple Imaging Polynomial convention to store polynomial distortion, the WCS Paper IV Lookup Table convention for residual distortion, the alternate WCS description to store catalog alignment information, and a binary table for book keeping purposes. This paper provides details on how these various conventions have been merged to create a unified comprehensive description of the WCS.